1. Field of the Invention
The present invention relates to a headup display apparatus and, more particularly, to a headup display apparatus in which display light is projected toward the windshield so that a driver can see a display image reflected according to the shape of the windshield. It is designed to prevent a phenomenon in which external light as from the sun travels through the path of the display light in a reverse direction to illuminate the surface of an indicator, making the display unrecognizable. The headup display is constructed in such a way that the display light radiated from the indicator is reflected by an off-axis reflecting member, such as hologram and prism, onto the windshield which, in turn, reflects the incoming display light toward the viewing point of the driver.
The present invention relates to a headup display apparatus and, more specifically, to a headup display apparatus which projects the display light onto the windshield which, according to the shape of its surface, reflects the display light toward the driver so that he or she can see it.
2. Description of the Related Art
In general, the headup display superimposes an image information on a background view, allowing a person to see a great deal of information simultaneously without requiring large body motions or large shifting of line of sight on the part of the driver. Because of this advantage, the headup display has already been put into practical use with cockpits of fighter and commercial aircraft. In recent years, the use of the headup display in automobiles has been considered.
In applying the headup display apparatus to automobiles, there are three types of combiners available for transmitting the background light and reflecting the display light. The first type employs a reflection type hologram formed integral with the windshield. As shown in FIG. 37a, a projection optical system unit 19 including an indicator is mounted in a dashboard 17 and the display light h from the unit is reflected by the reflection type hologram 5 formed on the windshield 15 to direct the light toward the viewing point 6 of a driver. The second type has a separate hologram combiner 5' installed on the dashboard. As shown in FIG. 37b, the projection optical system unit 19 including the indicator is installed inside the dashboard 17. The display light h is reflected by a hologram combiner 5' toward the viewing point 6 of the driver. The third type uses as the combiner a windshield not formed with a hologram or a windshield treated to increase the reflection factor. As shown in FIG. 37c, the projection optical system unit 19 including the indicator is mounted in the dashboard 17 and the display light h is reflected by the windshield 15 toward the viewing point 6 of the driver.
The first type requires making the reflective hologram in the manufacturing process of the windshield, and this increases the cost. Another disadvantage is that, when viewed from outside the car, the hologram reflects external rays of light and looks colored, marring the image of a high-class product.
The second type has some design problems in that the outline of the combiner exists in the viewing field in the windshield and that a structural member is placed on the dashboard.
On the other hand, the third type, though it has a problem of low display light utilization, has the advantage of low manufacturing cost. It is superior in design to other types and therefore is most promising for automotive applications.
FIG. 38 shows the action of the third type of combiner mentioned above. When sun light i travels along the light path of the display light h in the reverse direction and enters the projection optical system unit 19 from an opening 16 in the dashboard 17, it illuminates the indicator 1 which, when it uses a fluorescent light indicator tube, strongly disperses light because of a white color of the fluorescent substance, resulting in a phenomenon in which the display segments seem to light up regardless of whether the display is actually turned on or off, making the correct display impossible. FIG. 39 shows the condition of abnormal illumination. FIG. 39a represents a correct display condition indicating a number "80." The shaded part of the display segments is not lighted. FIG. 39b shows an abnormal illuminating condition in which the entire surface of the display segments is struck by a strong light (sun rays) which is scattered by the white fluorescent substance, making the display segments look as if they are emitting light. When the intensity of the scattered light is strong as compared with the display light (the sun rays in a clear day have sufficient intensity), it is difficult to distinguish between the correct and the abnormal display. The fluorescent display tube has a high level of luminance as compared with other types of indicators and is inexpensive, making it the most promising device as an indicator of the headup display. Under these situations, there has been a call for solution of the abnormal illumination of the fluorescent display tube.